1. Field of the Invention
The present invention relates to a wave-transmitting cover that covers the opening in the exterior member such as front grille of a vehicle with a wave radar such as a millimetric wave radar or a microwave radar mounted thereon in such a manner that the wave radar is on the back side of the cover, and to a method for producing the cover.
2. Related Art
An autocruise system is for a technique of controlling the distance between a driver's car and the one in front, for which a sensor mounted in the front part of the driver's car acts to measure the distance between the driver's car and the one in front and to measure the relative velocity between the two cars, and, on the basis of this information, the throttle or the brake of the driver's car is controlled so as to accelerate or reduce the speed of the driver's car. Recently, the autocruise system has become noticed as one core technique of intelligent transport systems (ITS) that are for reducing traffic jams and accidents.
For the sensor in the autocruise system, generally used are laser radars and wave radars. Of those, wave radars send microwaves having a frequency of from 300 MHz to 300 GHz and a wavelength of from 1 mm to 1 m to an object, and receive those having hit the object and having been reflected by it, thereby determining the distance between a driver's car and the one in front and the relative velocity between the two cars from the difference between the microwaves they have sent and those they have received. Of such microwaves, millimetric waves have an especially short wavelength, concretely having a frequency of from 30 GHz to 300 GHz and a wavelength of from 1 to 10 mm, and wave radars that utilize such millimetric waves may be down-sized. Down-sized wave radars have heretofore been well used for on-vehicle radars. In addition, since radio waves have a large reflection coefficient to good conductors such as metal, they are effective for discriminating vehicles from each other. Further, as compared with lasers, they are hardly influenced by fog, snow, sunlight, etc. Therefore, wave radars are favorable for on-vehicle radars.
In general, a wave radar is disposed on the back side of the exterior member of a vehicle such as a front grille. Of the exterior member thereof, however, the front grille or the like is plated with metal in many cases, and the metal-plated part could not well transmit radio waves that have a large reflection coefficient to metal. In addition, the front grille is specifically so constructed as to have air intake through-holes, therefore not having a uniform thickness. Accordingly, when radio waves are let in and out via it, then there occurs a difference in the wave transmission rate between the thick part and the thin part of the front grille and the wave radar, if put behind the front grille, could not exhibit a good sensitivity.
Given that situation, in general, a wave-transmitting opening is formed in a site of the exterior member of a vehicle that corresponds to the site where a wave radar is to be disposed. When such an opening is formed in the exterior member of a vehicle, then waves may be let in and out via it. However, the opening may detract from the outward appearance of the integrally-constructed exterior member, and, in addition, the inside structure of a vehicle that houses, for example, its wave radar and engine room therein may be seen through the opening, and it may further detract from the outward appearance of the vehicle. In the related art, therefore, a wave-transmitting cover part is inserted into the opening of the exterior member to thereby ensure the integrated appearance of the opening and the exterior member body (for example, see Japanese Patent Publications Nos. 2000-159039 and 2000-344032). Of the exterior member of a vehicle, a front grille is generally plated with metal, as so mentioned hereinabove. Therefore, the decorative face of the wave-transmitting cover must be processed in any manner to have a metallic gloss. In general, the metallic gloss is formed through indium vapor deposition. Indium may be deposited to form fine islands, and waves may pass through the space among the islands. Therefore, the indium vapor deposition enables both the metallic gloss and the wave transmission.
The radar wave-transmitting cover as disclosed in JP 2000-159039A or JP 2000-344032A is formed of a laminate of resin layers each formed to have a prismatic structure. In this, a metal-deposited layer is formed between the resin layers and it gives a decorative face of the cover. Thus designed, the cover may have an impression as if the fins formed in the front grille be still continued even in the cover part.
However, the radar wave-transmitting cover of the type has such a laminate structure of multiple resin layers and a metal layer formed through vapor deposition between the resin layers, and is therefore problematic in that it requires many steps and molds for molding it. In addition, it further requires a high-precision molding technique in order to flatten the radar wave-transmitting cover as a whole and to make it have a uniform thickness. Moreover, when the decorative face is formed on the prismatically-structured resin layer, it requires a process of vapor deposition and painting. For example, when the decorative face is formed through painting and vapor deposition, then the process for it comprises many steps of, for example, a masking step of masking the area except that to be painted, a panting step of painting the intended area, a drying step of drying the paint, a peeling step of removing the mask, another masking step of masking the area except that to be exposed to vapor deposition and a peeling step of removing the mask that are combined in that order. When the intended design for the decorative face is complicated more, then the number of the steps to constitute the process increases more, and it causes the increase in the production costs.
To reduce the thickness fluctuation that may be caused by precision molding errors, a metal layer may be formed separately and it may be laminated between resin layers in a mode of in-mold molding, and the resin layers may be formed in a mode of two-part molding (for example, see Japanese Patent Publication No. JP H09-239775).
According to the production method for transparent resin products disclosed in JP H09-239775A, the resin layers may be formed in a mode of two-part molding, and molded articles having a predetermined thickness may be obtained not requiring any high-precision molding technique. However, even the production method of JP H09-239775A still requires painting and vapor deposition for forming the decorative face, like in the above, and it is still problematic in that the production costs increase owing to the increase in the number of the process steps.